dynamic_ha_SUITE: Increase the chance of concurrent rebalances

... in `rebalance_multiple_blocked`.

The testcase wants two rebalances to happen at the same time: the first
one may succeed (but this is out of scope for this testcase) and the
second one must be aborted with `{error, rebalance_in_progress}`.

It looks like things are faster with Erlang 23 because the first
rebalance is already finished when the second starts, probably due to
internode communication being slower than rebalance (the two rebalances
are triggered from the common_test node).

This time, the testcase starts a function on the remote node which
spawns two functions in parallel to request a rebalance. Everything
being local, we increase the chance to have concurrent rebalances.

We don't know the order of execution of the two functions, so we simply
verify that one of them fails.

This is still not 100% bullet proof, but should be ok.

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